This invention relates to the field of metal alloys. More particularly, it relates to the field of amorphous aluminum- and magnesium-based alloys.
In the search for engineering materials researchers have targeted magnesium-based alloys as having potentially desirable properties. These desirable properties include exceptionally low density and excellent machinability at relatively low cost. However, magnesium also has properties frequently considered to be undesirable, including poor corrosion resistance, strength (particularly compressive strength), and formability. In view of this, much effort has been directed toward improving conventional casting and wrought alloys containing magnesium. In spite of this, magnesium-based alloys have never seen wide usage for structural applications because of their problems with corrosion resistance and mechanical properties.
In recent years extensive research has also been conducted on amorphous alloys. In general amorphous alloys are considered to be potentially more desirable because they exhibit improvements in various properties over crystalline alloys. These properties often include, for example, tensile strength, hardness, ductility, corrosion resistance, and magnetic properties including hysteresis loss and magnetoelastic effects. Transition metal/metalloid pairs, early transition metal/late transition metal pairs and transition metal/rare earth pairs have been well researched. In addition, some work has been done on amorphous alloys based on simple metals such as magnesium, calcium, strontium, zinc, copper, silver and cadmium. Simple metals as used herein refers to metals having only s or p valence electrons, thus excluding transition metals, rare earths, and actinides. Among these, the amorphous magnesium-based alloys prepared most often have been alloys of magnesium with copper, nickel, zinc, antimony, calcium or bismuth. However, the densities of most of these alloys, except for some of the calcium-containing alloys, are too high to make them of practical interest as a lightweight alloy.
For example, one publication describing magnesium-based amorphous alloys is F. Sommer et al., "Thermodynamic Investigations of Mg-Cu and Mg-Ni Metallic Glasses," J. de Phys., Colloque C8, Vol. 41, No. 8 (August 1980), 563-566. This work describes metalloid-free metallic glasses in the Mg-Cu system and the Mg-Ni system. In both of these systems glasses are formed at greater than 50 percent magnesium. F. Sommer et al., in "Formation Conditions and Thermodynamic Stability of Glassy Ternary Alloys," Proc. 4th Int. Conf. on Rapidly Quenched Metals (Sendai, 1981), 209-212, discusses ternary alloys based on the binary glass forming Mg-Cu and Mg-Ni systems. In this publication Mg-Cu-Ni, Mg-Cu-Zn and Mg-Ni-Zn ternary systems were investigated and found to form glasses along the entire length of the invariant valleys connecting the binary eutectics, which are surrounded by glass-formation regions. It is also noted that, in ternary systems where only one of the constituent binary systems exhibits easy glass formation, complete glass formation vanishes with very small additions of a third component (e.g., Mg-Cu-Sb, Mg-Ni-Al and Mg-Ni-Ag), or can be found only up to a certain concentration of the third component (e.g., Mg-Cu-Sn, Mg-Cu-Pb, Mg-Cu-Ag, Mg-Ni-Sn and Mg-Ni-Sb).
Alloys in the Mg-Zn system have also been widely investigated, for example, in A. Calka et al., "A Transition-Metal-Free Amorphous Alloy: Mg.sub.70 Zn.sub.30," Scripta Metallurgica, Vol. 11 (1977), 65-70. This work describes the Mg-Zn system as especially favorable for amorphous metal formation, based on the existence of a deep eutectic. The size difference of the components and their tendency to form complex equilibrium compounds combine to make this a promising system. Complete amorphization in the 68-75 atom percent magnesium range was observed.
Another Mg alloy system investigated is the amorphous Mg-Ca binary system. These alloys are described in, e.g., R. St. Amand et al., "Easy Glass Formation in Simple Metal Alloys: Amorphous Metals Containing Calcium and Strontium," Scripta Metallurgica, Vol. 12 (1978) 1021-1026. The alloys studied formed glasses over the range Ca.sub.57.5 Mg.sub.42.5 to Ca.sub.77.5 Mg.sub.22.5. Although such alloys should be very lightweight, they have in our experience been found to be very prone to oxidation when containing large proportions of calcium.
Another metal of great significance for lightweight structural uses is aluminum, which has a density about 1.5 times that of magnesium. Aluminum is very important commercially, due to its low density, good formability, oxidation resistance and high strength. These properties make aluminum a desirable alloy component for many structural applications. For some of these uses, however, it is desirable to have an even lower density material. In the quest for lower density alloys, aluminum has therefore been combined with lithium. See, e.g., T. H. Sanders et al., "Aluminum-Lithium Alloys: Low Density and High Stiffness," Metal Progress, Vol. 113, No. 3 (1978), 32-37. In this combination the few percent of lithium reduces density significantly and increases the modulus of elasticity of the resulting alloy. However, the amount of lithium which can be added is limited, and this thereby limits the amount of density reduction attainable.
Amorphous aluminum alloys have also been prepared. For example, Y. He et al., in "Synthesis and Properties of Metallic Glasses That Contain Aluminum," Science, Vol. 241, 1640-1642, disclose the synthesis of metallic glasses containing up to 90 atom percent aluminum. These glasses exhibit unusually high strength. The glasses described are mainly ternary glasses based on an Al-Fe system. Aluminum-based amorphous alloys are also described by a research team at the Institute for Metals Research of Tohoku University in Japan. According to an article, "Extra Strength Claimed for New Aluminum-Based Alloy," in Metalworking News, June 6, 1988, p. 26, small amounts of nickel, yttrium, lanthanum and other rare earths are added to produce an alloy that is twice as strong as super-duraluminum.
Attempts to reduce the density of conventional aluminum alloys by alloying aluminum and magnesium have also been envisioned, analogous to the production of aluminum/lithium alloys. However, those skilled in the art expect serious problems in this endeavor, because these two elements have only limited solid solubilities in each other. Incorporating too much of either element into a crystalline alloy with the other results in the formation of excessive amounts of intermetallic compounds, which exhibit very undesirable mechanical properties including brittleness. Nevertheless, despite these problems it is still potentially desirable to alloy the two together because the magnesium has the potential to impart to an alloy a lower density than aluminum, while the aluminum has the potential to offset the poor corrosion resistance, compressive strength and formability of magnesium.
Thus, despite research done on magnesium-based and aluminum-based crystalline and amorphous alloys, there is still a need for improved alloy systems utilizing magnesium and aluminum. Preferably these systems should combine the positive features of magnesium, including its low density and excellent machinability, with the high tensile and compressive strength, ductility, oxidation resistance and corrosion resistance of aluminum. Even more preferably, these systems should also exhibit the positive features of an amorphous alloy, including higher strength and ductility.